With an increase in the density of LSI, the circuit line width required for semiconductor devices has decreased through the years. To form a desired circuit pattern for these semiconductor devices, an original image pattern (also referred to as a reticle or a mask) with high precision is needed. Such an original image pattern with high precision is produced by using an electron beam lithography technique with an electron beam writing apparatus.
Examples of the electron beam writing apparatus include a multi-beam writing apparatus with an increased throughput by applying a large number of beams at a time with a multi-beam technique. In this multi-beam writing apparatus, for example, multiple beams are formed by letting an electron beam emitted from an electron gun pass through an aperture member having a plurality of apertures, and blanking control is performed for each of the beams. Unblocked beams are diminished in an optical system and applied onto a desired location on a mask to be written. The period of application of a beam is controlled for each pixel area (pixel) onto which one beam is applied.
In the case of performing pattern writing with gray scale control in consideration of the spread of electron beams or correction of line width variations due to, for example, a proximity effect in the electron beam writing apparatus, to control an irradiation amount distribution with high precision, the irradiation time assigned to each pixel area has fractional portions shorter than a minimum control unit period that can be controlled with the writing apparatus. It is technically difficult to reduce the minimum control unit of the irradiation time in the writing apparatus, and thus, electron beams cannot be applied onto each pixel area for a desired irradiation time. Consequently, it is difficult to increase the writing precision.